Well, this page (click) says that Jack Thorp's papers are in Santa Fe - but apparently not available online. Hey, Santa Fe is a wonderful place to visit....
Here's what's in the collection:

Collection consists of the papers of Nathan Howard "Jack" Thorp. The bulk of the collection consists of undated cowboy songs, poems, riddles, and short stories. Included are New Mexican folk songs and documents pertaining to New Mexico saints and feast days. Also within the collection is a letter concerning editing of Thorp’s manuscripts (1936).

I don't have the Fifes' book on Songs of the Cowboys. What does the Fife book have, that the Thorp book does not?
-Joe Offer-